1. Technical Field
The present invention relates to a terminal apparatus which uses a signal from a positioning satellite for positioning, a control method for the terminal apparatus, a control program for the terminal apparatus, and a computer readable storage medium having stored therein the control program for the terminal apparatus.
2. Related Art
A positioning system that positions the current position of a GPS receiver by using, for example, a GPS (Global Positioning System) which is a satellite navigation system has conventionally been put into practical use.
A GPS receiver, for example, selects four GPS satellites that are observable at the current time, predicts receiving frequencies from the respective GPS satellites, and receives signals from the respective GPS satellites. Based on the received signals, the GPS receiver determines the distance (hereinafter referred to as the “pseudo-range”) between each GPS satellite and the GPS receiver by the difference (hereinafter referred to as the “delay time”) between the time when a signal is transmitted from a GPS satellite and the time when the signal reaches the GPS receiver. Then, the GPS receiver calculates the position of each GPS satellite at the current time using orbit information (hereinafter referred to as the “ephemeris”) on each GPS satellite and then performs a positioning operation on the current position using the respective orbital positions of the GPS satellites and the aforementioned pseudo-range. By the positioning operation, a positioning position in latitude, longitude, and altitude, for example, can be obtained.
In the aforementioned positioning using a GPS, when, taking into account the frequency error of a local oscillator of a GPS receiver, a wide receiving frequency range is set, the reception of signals from GPS satellites requires a long time.
In relation to this, there is proposed a technique which uses a feature that, in a terminal apparatus into which a mobile phone and a GPS receiver are integrally formed, the mobile phone is in synchronization with the carrier frequency of a communication wave from a base station (for example, U.S. Pat. No. 5,841,396). According to such a technique, when a mobile phone is in synchronization with the carrier frequency of a communication wave, by using a feature that the frequency of an output signal from a VCO (Voltage Controlled Oscillator), for example, which is a local oscillator that generates a reference signal (or a reference clock) of the mobile phone has accuracy that conforms to the carrier frequency of the communication wave, the frequency error of the reference signal (or reference clock) which is reference for the operation of a GPS receiver can be calculated and then the frequency error of the local oscillator of the GPS receiver can be corrected.
However, in a method of achieving synchronization with the carrier frequency of a communication wave by using, as a reference oscillator which generates a reference clock of a mobile phone, a TCXO (Temperature Compensated X′tal Oscillator) having stable accuracy, for example, and by using, as a digital oscillator, an NCO (Numerical Controlled Oscillator), for example, the aforementioned technique cannot be used in this method because the frequency of the reference oscillator is not corrected.